Ask the Doctors

Dear Doctor: Colorectal cancer is rising among young people, as I recently read in a news report. But why? I thought colorectal cancer required a lifespan of unhealthy behavior.

Dear Reader: When I went through medical school, my professors had the same notions about colon cancer. The prevailing belief was that colon cancer was predominantly a disease of those over 50. Yes, colorectal cancers did occasionally occur in younger people, but these cases were attributed to familial diseases such as Lynch syndrome or familial adenomatous polyposis, or to a significant family history of colon cancer. Ulcerative colitis or extensive Crohn's disease could also put younger people at risk of colon cancer.

However, data have been warning for several years now of a rise in colon cancers diagnosed prior to the age 50. A 2015 study in JAMA Surgery looked at colorectal cancer data from nine states between 1975 and 2010. During that time, the overall rate of colon cancer decreased -- but only because the rate of colon cancer after the age of 50 decreased. In that study, 92 percent of colorectal cancer cases occurred after age 50; 8 percent occurred before age 50; and only 1 percent occurred between the ages of 20 and 34. In that same study, the 35-to-49 age group experienced little change in the colon cancer rate, but among those ages 20 to 34, the rate of colorectal cancer cases increased by 2 percent per year.

When the researchers looked specifically at localized (meaning they hadn't spread) rectal cancers and sigmoid colon cancers in this younger age group, they found that the rate of these cancers had increased by 4 percent. Because the total numbers of colorectal cancers in this age group was small, the increased rate of cancer in that age group was also small. But the authors noted that over the next 20 years, the rates may continue to increase at the current rate, and a substantial number of cancers, especially in the rectal and sigmoid areas, may be diagnosed among people ages 20 to 34.

A 2017 study looked at similar data from the same nine states between the years 1974 and 2013 -- and had similar findings. As expected, the rates of colorectal cancers decreased in those over 55 and decreased even further in those over 70. Among people ages 20 to 40, the cancer rates decreased between 1974 and 1985, but then substantially increased between 1985 and 2013. Between 1985 and 2013, people ages 30 to 39 had a rate increase of about 1 percent, and those 20 to 29 had a rate increase of 2.4 percent. Again, the absolute rate of colorectal cancers in this age range is low, such that the incidence increased from 1 in 200,000 people per year to 1 in 100,000 per year.

But why the increase? Consider that the increased rates of colorectal cancers in this younger age group coincide with increased rates of obesity among young Americans, with multiple studies confirming the correlation. Further, colorectal cancer has been linked to decreased consumption of fruits, vegetables and fiber and an increased consumption of processed meats -- characteristics of a modern American diet reliant upon processed food.

Although the rates of colorectal cancer in younger Americans are still relatively low, the increased incidence in this age range is emblematic of unhealthy habits among younger Americans. If those habits don't change, we can expect the colorectal cancer rates to keep rising.